As series boss / writer Steven Moffat explained: "It's the 12th Doctor saying to Number One, 'Listen, you have to [change] or all of this stuff won't happen!' and the 1st Doctor saying, 'Well, what about you?' – they go off on a crazy adventure with Mark Gatiss and Bill Potts (Pearl Mackie) and decide whether or not they're going to carry on... which of course, they are!"

Doctor Who will return to BBC One and BBC America this Christmas for Capaldi's (and Moffat's) swansong, before returning with a new Doctor and a whole new team in 2018.